The present invention relates to a dielectric ceramic and a dielectric device for use in high-frequency regions such as microwave regions and millimeter-wave regions.
Recently, dielectric ceramics for microwave regions and millimeter-wave regions have been used widely in dielectric resonators and filters. It is required that a dielectric material used for such purposes has a high unloaded Q (Qu) value and a high dielectric constant (xcex5r), and that the temperature coefficient (xcfx84f) of a resonant frequency is small and variable as desired.
Various materials appropriate for use in such applications have been reported. The examples include a ZrO2xe2x80x94SnO2xe2x80x94TiO2xe2x80x94MgO ceramic disclosed in JP-62(1987)-132769 A and a ZrO2xe2x80x94SnO2xe2x80x94TiO2xe2x80x94CoOxe2x80x94Nb2O5 ceramic disclosed in JP-02(1990)-192460 A.
However, the conventional materials suffer from problems, for example, the unloaded Q value and/or the dielectric constant is low, or a desired temperature coefficient of a resonant frequency cannot be realized.
Though the applicant suggests a Zrxe2x80x94Tixe2x80x94Mn-based dielectric ceramic in JP 2768455 (U.S. Pat. No. 5,356,843), the average grain size in this method exceeds 100 xcexcm and the mechanical strength is not sufficient.
Although the product of a resonant frequency (f) and a Qu value is generally regarded as being constant in a given material, actually, the product fQu is reduced considerably when a bigger device is produced and the resonant frequency is lowered. Therefore, there is a strong demand for, e.g., a dielectric resonator for a base station of a mobile radio communication system to have a high Qu value.
Moreover, since defects such as chips and fractures will occur often when the mechanical strength of a dielectric ceramic is low, the mechanical strength of the dielectric ceramic should be increased in production of such devices.
The cost for polishing is increased among the whole cost of producing the dielectric device, and precision in adjusting the frequency cannot be satisfactory.
Since dielectric resonators used in the relatively-low frequency regions of 0.4 GHz to 2.4 GHz tend to be very bulky, reduction in size is highly demanded.
A first object of the present invention is to provide a dielectric ceramic having a high Qu value and xcex5r even in a relatively-low frequency region so as to realize a desired xcfx84f, and the dielectric ceramic has an improved mechanical strength as a result of optimizing the average grain size. A second object of the present invention is to provide a cost-effective and highly precise method for adjusting the frequency. Furthermore, a third object of the present invention is to provide a small dielectric device having a high Qu value in a frequency region of 0.4 GHz to 2.4 GHz.
For achieving the above-described objects, a dielectric ceramic according to the present invention includes a sintered body of a complex oxide comprising at least one element selected from the group consisting of Zr, Ti and Mn, at least one element selected from the group consisting of Mg, Zn and Co, and at least one element selected from the group consisting of Nb and Ta, wherein the complex oxide is represented by a formula of xZrO2-yTiO2-zA(1+w)/3B(2xe2x88x92w)/3O2 where xe2x80x98Axe2x80x99 in the formula denotes at least one element selected from the group (A) consisting of Mg, Zn and Co, xe2x80x98Bxe2x80x99 denotes at least one element selected from the group (B) consisting of Nb and Ta; x, y, z and w denote values respectively in the ranges of 0.20xe2x89xa6xxe2x89xa60.55, 0.40xe2x89xa6yxe2x89xa60.55, 0.05xe2x89xa6zxe2x89xa60.25, 0xe2x89xa6wxe2x89xa60.30, and x, y and z have a relationship represented by x+y+z=1; MnO is present in a range of 0.1 mol % to 1.0 mol % to the complex oxide; and an average grain size of the dielectric ceramic is from 10 xcexcm to 70 xcexcm.
A method for producing a dielectric ceramic according to the present invention includes calcining a material containing at least one element selected from the group (A) consisting of Zr, Ti, Mn, Mg, Zn, and Co, and at least one element selected from the group (B) consisting of Nb and Ta; pulverizing, adding a binder and press-molding the material to form a molded product, heating the product to remove the binder, and firing the product at a temperature of 1200xc2x0 C. to 1400xc2x0 C., subsequently annealing the product at a temperature lower than the firing temperature by a range of 50xc2x0 C. to 100xc2x0 C.
Next, a dielectric device according to the present invention includes a metal housing and a dielectric ceramic placed in a cavity of the metal housing, wherein the dielectric ceramic is made of a sintered body of a complex oxide containing at least one element selected from the group consisting of Zr, Ti and Mn, at least one element selected from the group consisting of Mg, Zn and Co, and at least one element selected from the group consisting of Nb and Ta, in which the complex oxide is at least one element represented by a formula xZrO2-yTiO2-zA(1+w)/3B(2xe2x88x92w)/3O2 where xe2x80x98Axe2x80x99 in the formula denotes at least one element selected from the group (A) consisting of Mg, Zn and Co, xe2x80x98Bxe2x80x99 denotes at least one element selected from the group (B) consisting of Nb and Ta; x, y, z and w denote values in the respective ranges of 0.20xe2x89xa6xxe2x89xa60.55, 0.40xe2x89xa6yxe2x89xa60.55, 0.05xe2x89xa6zxe2x89xa60.25, and 0xe2x89xa6wxe2x89xa60.30, and x, y and z have a relationship represented by x+y+z=1; MnO is present in a range of 0.1 mol % to 1.0 mol % to the complex oxide, and an average grain size of the dielectric ceramic is from 10 xcexcm to 70 xcexcm.